Elipetical Birds mouth mast

Discussion in 'Sailboats' started by Chuck Losness, Feb 8, 2016.

  1. Chuck Losness
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    Chuck Losness Senior Member

    I must have too much time on my hands. Any way I have often wondered if you could make an elipetical shaped birds mouth mast. In playing around with different shapes I discovered that using a 2-3-4 ratio of length with 30/60 degree birds mouth cuts gives you an rough elipetical shape where the length is 2 times the width. For example a mast with pieces that are 2", 3" and 4" wide gives you a mast that is roughly 10" x 5". I have no idea what loads this mast could carry. You would have to figure out how to do the calculations. My intuition suggests that a stiffener would be needed to keep the mast from buckling. Any way I just thought that I would share this with the forum.
     

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  2. PAR
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    PAR Yacht Designer/Builder

    I've done this. Generally, you work in reverse and design the mast for the anticipated loads. If appropriately stayed, you wouldn't need spacers. If freestanding, keep it round.
     

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  3. Mr Efficiency
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    Mr Efficiency Senior Member

    I must take closer note of what bird's mouths look like, oh I think I get it now.....
     
  4. HJS
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    HJS Member

    A free-standing mast must be dimensioned so that it bends in such a way that the sail has the right camber at different loads. The attached sketch shows the mast to a Swedish E-canoe with 13 square meters of sail. It is never to be completely circular.

    js
     

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  5. PAR
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    PAR Yacht Designer/Builder

    I understand JS and that mast section shows very light scantlings. Is that freestanding? Given it's 15% wall thickness (my approximation), I'd have to guess it's a stayed mast.
     
  6. gonzo
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    gonzo Senior Member

    Rotating freestanding masts can be oval or streamlined
     
  7. TANSL
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    TANSL Senior Member

    Totally agree. In this case, the mast should not be circular.
     
  8. PAR
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    PAR Yacht Designer/Builder

    Again, agreed, but adding to the equation isn't really helpful. The original questions were can an elliptical or oval mast in birdsmouth be done. Knowing this particular OP's interests in freestanding I made an assumption in that regard, but no mention of rotation was made. I possibly incorrectly assumed a fixed, possibly freestanding birdsmouth stick.
     
  9. TANSL
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    TANSL Senior Member

    I agree but I think one of the advantages of free standing mast is that it can be rotating. Therefore, in my view, a fixed free standing mast, makes little sense, except in a very small boat. Here I would like to know the views of experts.
     
  10. HJS
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    HJS Member

    The masts I refer to are rotating. They are rigid in transverse direction in the lower part and the rigid longitudinally in the upper part. They must also be dimensioned according to the helmsmans weight. The technology is well proven in the Finn class and on our sailing canoes.

    JS
     
  11. gggGuest
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    gggGuest ...

    Worth reiterating that those Kanot/Finn masts are very sophisticated racing kit, if a tad heavy by modern carbon standards. I must admit miy mind boggles when I think of the work in getting the pieces right for that varying section. However think the sketch is v useful for demonstrating well established technology going past what Op had in mind.
     
  12. Chuck Losness
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    Chuck Losness Senior Member

    My interest is in freestanding rotating masts. Birdsmouth spars look to be relatively easy to assemble. I don't know what the weight would be. Might be too heavy. I have not tried to modify my spreadsheet to calculate the loads on this particular birsdsmouth mast. I might or might not do that.
    I am actually looking into two options for my last boat. One option is a 25 to 28 foot cat ketch trailerable sailboat. I do not have the funds to make a custom boat so I would have to modify an existing design. The Beachcomber 25 comes very close to what I am looking for in a sailboat. Not many were made and I have never seen one on the west coast. The other option I am looking into is a small trailerable displacement powerboat. Most likely a conversion of a sailboat to strictly power with a 9.9hp high thrust outboard as the primary engine. Google terminal trawler or strawler and you will see what I am considering.
    Either boat would have to be able to be launched on a standard boat ramp.
    The romantic side of me says go for the sailboat. The practical side says go for the powerboat. And where I will end up is anybodies guess.
     
  13. messabout
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    messabout Senior Member

    Chuck, Birdsmouth assemblies are not as easy at it looks. Sure enough it is an attractive method but assembling all those long sticky, sometimes tapered sticks is a bear to get into their proper places and clamped up with good fits at every contact surface. I have built a few of them and I suggest that it does lead to spontaneous utterings not fit for Sunday school.

    If the mast is to be free standing, a perfectly fitted plug needs to be installed at the high pressure locations. If it is to be supported by shrouds, then a perfect plug needs to be hidden inside at the shroud attachment points. All that is do able but it demands precision in the birdmouths grooves and every place else. The 90 degree groove is best offset a predetermined amount in order to leave the joining surfaces as wide as possible and to reduce the amount of planeing that makes the surfaces fair.

    Aside from all that, the structure is an elegantly engineered assembly that is light and it can be strong if the segments are thick enough.
     
  14. PAR
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    PAR Yacht Designer/Builder

    Most find the first one to be an eye opening experience. What I took away from my first birdsmouth was to make the stick in halfs and to make stands, spaced along the length, to control staves during the glue up. The next mast I did, these two things helped greatly and I discovered that bicycle inner tube cut into strips, was the best stuff to clamp the mast closed with. The next mast proved that the asymmetric stave layout was easier to round than the symmetric and that hard points (swallowtails) are easier cut as 1/4's instead of 1/8ths, etc., etc., etc. I invented new and deviously clever cuses on my first birdsmouth build, but now have it down to a system that makes them easy. Naturally, the slowest goo you can get is helpful and a lathe to round it, is also a God send. My "new" lathe is a electric motor driving a garden tractor 4 speed transmission, which takes the 3400 RPM motor down to just a bit too fast or just a bit too slow, with two other speeds that are just right.
     

  15. LP
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    LP Flying Boatman

    On the birds mouths that I have done, when I did the dry fit, I wrapped the spar at intervals with packing tape. When I was ready to glue up the spar, I split the tape along one stave joint to unroll it. I applied the glue and rolled it back up and clamped it up with more tape.
     
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